When Alex and Conner try to find Snow White's glass coffin at her palace, Snow White tells them she gave it back to the dwarfs and it's in the dwarf mines.

"The twins found a long, miniature table with a few dozen miniature chairs around it. It looked like an area where the dwarfs took breaks from working. A large portrait of Snow White hung on the dirt wall behind it, and a glass coffin with rubies and diamonds sat against the wall underneath it."[2]

When they take a few, there are already other jewels missing from the coffin.

Apart from being an ingredient for the spell, it does not say if the jewels have any other magical qualities.